The present invention relates to a device for extending the run time of an air conditioning or forced-air furnace fan after the compressor or furnace stops and thereby increasing the cooling/heating output and in the case of air conditioning, increasing the Sensible Energy Efficiency Ratio (SEER).
Heating consumption accounts for 41% of total residential energy use in the United States as reported by the US Energy Information Agency Residential Energy Consumption Survey in 2005. Known central heaters are controlled by a thermostat which turns on a heater ventilation fan after a brief delay following turning on a heat source, and turns off the heater ventilation fan after variable temperature based delay or a fixed time delay following turning off the heat source. Unfortunately, maintaining a low heater ventilation fan speed often results in increased heat soak within the central heating unit and the portion of the heat generated by the heat source is lost to the environment increases the longer the central heating element is on at the low heater ventilation fan speed. Further, the amount of heat soak increases as the central heating unit is operated for longer periods of time leaving significantly higher temperature air (i.e., 110 to 200° F.) in the heat exchanger after the heater ventilation fan is turned off and a portion of this heat is also lost to the environment after the heat source and the heater ventilation fan are tuned off.
U.S. Pat. No. 5,142,880 (Bellis, 1992) discloses a solid state control circuit for use in connection with existing low-voltage thermostat terminals of a central, forced-air, air conditioning system having a compressor and an indoor blower and gas-fired or electrical heating elements. The '880 patent relates generally to systems for increasing the efficiency of air conditioning units by continuing the blower running time after the compressor is turned off. Specifically, the '880 patent claims an air conditioning control unit comprising a low voltage room thermostat fan terminal, a low voltage compressor relay terminal, a timing circuit means, a sensitive gate triac, and a power triac. The '880 patent also claims a method for controlling the on-off time of an indoor fan that is controlled by and associated with an indoor thermostat for a room air conditioning system. The apparatus of the '880 patent is not programmable or adaptable. It does not have a fixed delay from one system to another. The delay is related to the supply voltage, which varies from system to system. Bellis provides constant current to the triac gates on the order of 6 milliamps. The total current draw is even higher than that when all components are included. Many systems have do not accommodate this much current draw through control relays without causing a humming noise which irritates the user. The Bellis design momentarily de-energizes the relay when switch from thermostat driven fan to his delay. This can cause relay chatter and excessive wear. Bellis does not provide for an override function if the unit fails. The Bellis design is a “fixed” delay. The delay does vary with respect to supply voltage, component variation and temperature. Bellis outlines between 3 and 5 minutes for the delay. His device is not programmable or adaptable.
In U.S. Pat. No. 5,882,233 Noto teaches of a device used to extend the fan run time and also periodically activate the fan during times the system is not calling for heating or cooling. Noto requires the circuit to have access to the 24 VAC signals from the AC transformer. This requirement precludes his device from being connected directly to the thermostat since most thermostats do not have both the hot and neutral legs of the transformer. Household wiring only provides the hot (red) signal to the transformer. Although Noto teaches of a range of delays, his invention uses fixed values for the delays. The delays in his invention are not programmable either by a user interface or by system conditions such as the duration of the compressor activation.
U.S. Pat. No. 4,842,044 (Flanders et al., 1989) provides a heating and cooling control system that works by energizing a fan or other fluid circulating device to circulate fluid and effect thermal transfer of energy from the fluid to the spaces being heated and by de-energizing the circulating means at a selected time interval after de-energization of he heating and control system. The '044 patent also claims a heating control system comprising a switching means to effect energization of the fluid circulating means, a switching control means that is energizable in response to operation of the control circuit, and an additional circuit means that energizes the switching control means a selected time interval after de-energization of the heating system. The '044 patent is intended to increase the time the fan is turned on after a heating cycle to improve energy efficiency. Unfortunately, it draws power continuously from the gas solenoid through a 680 ohm resistor, and this method has proven to be problematic in practice. Too much current drawn in this way, can cause a humming noise in the gas valve and even false operation. The patented device also enables the fan relay to activate the blower as soon as the gas valve is activated. This results in cool air being circulated throughout the home since the plenum is not sufficiently warm. Normal heat operation retards the blower until the temperature in the plenum reaches a preset operating temperature. The '044 patent also requires the addition of a relay circuit. This relay must be active the entire time the fan is to be off, creating a significant current draw even when the system is in not calling for heating or cooling. The'044 patent also describes fixed delays. It has no way to adapt the fan delay times either by user input or by the compressor run time. The delays provided by the '044 patent are also subject to the variations of the components selected. Additionally, although Flanders touches on the subject of how his invention works when the fan switch on the thermostat is moved from the AUTO position to the ON position, as described, there is no way for the fan to come on when the homeowner requests.
In U.S. Pat. No. 4,136,703, Kinsey teaches of a device that intervenes with the controls coming from a thermostat and going to the heating/cooling system. His invention places a fixed upper limit to the time that the compressor or heating source can be activated and then his invention adds additional time to the blower fan. This activity can increase the efficiency of an air conditioner system by allowing a certain amount of water to condense on the evaporator coil and then re-evaporating this water to cool the home. The amount of water collected will vary based on the humidity of the ambient air. Having a fixed compressor run time with a fixed blower time can create a less efficient system than the current invention. In many environments, limiting the compressor run time and counting on evaporative cooling to reduce the home's temperature will increase the time required to cool the home. In many cases, the desired set point may never be achieved.
In U.S. Pat. No. 7,240,851 Walsh teaches of a furnace fan timer. His device is strictly a timer with a user programmable interval and duration. His device runs continuously in a never ending loop counting down minutes before operating the fan and then counting the minutes to keep the fan activated. Walsh's device is not compatible with air conditioner systems. Most thermostats connect the fan switch to the air conditioner compressor switch when operating in the automatic fan mode. In systems with air conditioners, Walsh's invention will activate the air conditioner compressor when it turns on the fan. This requires users to turn off the circuit breakers for their air conditioner systems when using his device. Walsh's invention has two interchangeable wire connections. The present invention has at least three wires and each connects to a specific location on the thermostat.
U.S. patent application Ser. No. 13/085,119 filed Apr. 12, 2011 by Walsh and incorporated by reference into the present patent application discloses a system and method for extending fan operation in cooling mode but is not optimized for heating modes.